1. Field of the Invention
This invention relates to a system and a method for determining the flow of blood in blood vessels using ultrasound and a contrast agent that is introduced into the blood stream.
2. Background Art
There is a well-known need to determine blood flow in a patient. Lower than expected flow may, for example, indicate the presence of a thrombosis or of arteriosclerosis, and sudden changes in flow, corresponding to changes in cardiac output, are important indicators in patient monitoring.
There are, accordingly, many systems for measuring blood flow. Many common systems are catheter-based, and involve injecting an indicator upstream where the flow is to be measured and then measuring the indicator concentration at a downstream position. The time of flight then determines the average flow velocity. One typical indicator is some bolus of cold fluid or marker dye. Another common indicator is heat; in such systems, a heating element is used as the upstream indicator injector and a thermistor is used as the downstream sensor.
The obvious disadvantage of these methods is their level of invasiveness--by definition, catheter-based systems involve placing a catheter into the blood vessel. For monitoring cardiac output, for example, the catheter is commonly inserted into the femoral artery, that is, below the hip, and is threaded all the way up into the right pulmonary artery adjacent to the heart. Another disadvantage is that it is not always practical or even possible to reach certain flow measurement points.
Still another shortcoming of bolus-based systems is that it is usually not possible to carry out several measurements, at least not close to each other in time. One cannot, for example, inject a long series of cold (often near zero degrees Celsius) boluses into the patient's heart without causing more severe problems than the patient may have had in the first place.
Another known method for measuring flow velocity uses ultrasound. In systems that implement this method, ultrasound generated by a transducer is focused on a region of the flow, the received echo signal is sensed by the transducer, and flow velocity is calculated as a function of the sensed Doppler shift of the received signal relative to the transmitted signal. Current practical techniques or ultrasound flow velocity estimation using such Doppler methods are, however, not sufficiently quantitative. Moreover, Doppler techniques measure the component of velocity in only a single direction.
Yet another method that has been proposed uses ultrasound together with a contrast agent that is injected into the blood stream in the region of interest. According to this approach, a high-intensity ultrasound field is used to destroy the microbubbles in the contrast agent at some upstream point in the flow. The resulting gap or "negative bolus" in the contrast agent is then tracked downstream. Measurements of the transit time from the point where the microbubbles in contrast agent are destroyed to the final measurement point then provide quantitative estimates of flow speed.
A related approach involves interrupting the high-intensity ultrasound field, which is used to destroy the contrast agent over a period of time. This then creates a "positive bolus" of contrast agent, whose progress is tracked as it passes downstream.
The main problem with these techniques is that they give only a single measurement of the time of flight of the contrast agent bolus. This makes it impossible to track changes in the flow velocity. Of course, one could simply repeat the single-bolus measurement later, using several independent boluses, but it is then uncertain whether any given measurement was taken during a period of unusually high noise, such as often arises when the patient is being artificially ventilated.
Yet another problem with single-bolus techniques is that the accuracy of the measurement depends on how well the system can identify the edges of the bolus. Even if the ultrasound beam could create an initial bolus with perfectly straight edges, that is a step-function input signal, then the edges will invariably be rounded and spread out as the bolus progresses downstream. It then becomes difficult to determine just which point of the sensed bolus is to be matched with which point of the input signal. The closer together the input and output are, the more sensitive the speed calculation will be to such errors. On the other hand, the farther apart the input and output are, the more the bolus will be deformed before being sensed at the output,
What is needed is therefore a minimally invasive method that would make possible continuous or at least nearly continuous measurement of flow in order to track changes over time. The method should reduce the influence of temporary or non-representative disturbances such as ventilation noise, and it should be less sensitive to channel disturbances such as mixing and spreading than single-bolus or independent-bolus systems. This invention provides such an improved method, and a system to implement it.